The present invention relates generally to nondestructive testing of composite materials or panels, particularly wood based materials, such as plywood, oriented strand board, wafer board, particle board, and the like, to determine the strength and stiffness of such panels.
The use and acceptance of composite materials and panels for various applications, such as, building constructions, continues to increase in the market place. As a result, it is becoming increasingly desirable to monitor the strength and stiffness of the panels being produced. This is so because the strength and stiffness of composite materials varies greatly due to the composite nature of the products and the difficulty in achieving uniform strength in the bonding materials used to join the composites together. Moreover, variations in feedstocks and other factors make manufacture of uniformly strong and elastic structures from composite elements difficult and costly.
Nondestructive inspection and testing of materials of all sorts is known. Many of the known methods for performing certain standards tests are manual or static methods. For example, to conduct a concentrated load test, it is known to build a frame with beams simulating joists in a building construction. The beams are spaced apart depending upon the end use and span rating of the panel to be tested. A hydraulically-actuated load is applied to the stationary panel at a specified distance from a non-secured edge and the deflection of the panel is measured by placing a dial-micrometer under the panel at a position opposite the load and reading the deflection on the micrometer scale.
U.S. Pat. No. 4,708,020 to Lau et al., which is incorporated herein by reference, relates to another form of nondestructive inspection and testing of composite panels to determine the strength and stiffness of the panels. More particularly, Lau et al. provide an apparatus and process for correlating end-use strength and stiffness values when the testing is carried out on hot panels. The panels may be tested at one temperature, approaching the press temperature, and the strength and stiffness determined for the end products at another temperature, generally ambient or end-use temperature. Lau et al. also provide a testing machine suitable for in-line testing for determining the strength and stiffness of panel products having different thicknesses. The testing machine of Lau et al. also enables panels to be graded so that rejects can be identified and panels can be separated into grade groups representing different strength and stiffness ranges.
The continuous panel tester of Lau et al. imposes a double reverse bend or xe2x80x9cSxe2x80x9d shaped configuration on the panels as they pass through the conveyor at line speed. The device of Lau et al. is configured and operated such that either the deflection of each panel may be measured for a specific load, or the load is measured for a particular deflection of each panel.
As set forth in Lau et al., there is provided a first in-feed roll and a last out-feed roll to direct each panel to be tested into and out of the overall continuous panel tester and grader. As also described in Lau et al., a plurality of photo switches along the conveyor line have the function of informing the microprocessor when a panel is in the tester. The photo switches of Lau et al. determine when one panel ends and a second panel commences to pass through the tester so as to ensure that readings from the load cells and temperature sensor represent strength and stiffness figures for one panel. Another feature of Lau et al. is the ability of the panel grader to test panels having different thicknesses by merely selecting the required nominal panel thickness. The microprocessor is programmed to control the necessary equipment to position the rolls of the apparatus to process the panels of the selected nominal thickness. Based on the selected nominal thickness which is inputted to the microprocessor, the microprocessor utilizes information received from the load cells and temperature sensor to calculate the hot strength and stiffness values for each panel and then the microprocessor uses a preprogrammed algorithm to determine the ambient or cold end-use strength and stiffness value for each of the tested panels. Lau et al. do provide that it may be desirable to use a thickness measuring sensor such as a laser sensor or an ultrasonic sensor, which is placed near the in-feed rolls of the panel tester, to obtain a more actual thickness measurement of each panel, as compared to using the selected nominal thickness for each panel, thereby providing for a more accurate calculation of the strength and stiffness properties for each panel.
Despite the increased use of composite materials for all sorts of building constructions and other uses, and the general desire to test the composite materials for strength and stiffness, a need still exists for an improved panel tester and grader which is efficient and economical in its manufacture and use and which also provides improved accuracy in terms of measuring and grading panel like products according to desired strength and stiffness values.
As can be appreciated by those skilled in the art, the many known manual methods for performing certain standards tests for panels or the like are generally labor intensive, slow processing, somewhat costly procedures that can readily lead to error or operator mistakes when trying to determine the strength and stiffness values for panels. Moreover, the known static testing machines do not allow a panel to continually move along the production line during testing, thereby limiting the usefulness of such testing equipment.
Although Lau et al. describe an automatic, continuous panel tester and grader which is in many aspects an improvement over the known manual or static methods, the device of Lau et al. also exhibits several problems. One problem with Lau et al. concerns the bending forces that are applied to the panels as they are fed to and passed out of the panel tester. Although Lau et al. recognize that no significant forces should be applied to the panels that would distort the loading forces of the panels in the xe2x80x9cSxe2x80x9d shaped path, it has actually been determined according to the present invention that the first in-feed roller (40) and the last out-feed roller (70) of Lau et al. (see FIG. 2 thereof) do in fact apply undesirable bending forces or moments to the panels as they travel thereover, thereby resulting in significantly less than accurate strength and stiffness values for the tested panels. It has been determined according to the present invention that if the panels are subjected to a bending force outside the critical load zone or path, the deflection for a specific load or the load applied for a particular deflection may be greater than or less than what the actual deflection or load would be absent the undesirable bending force, depending on the direction the panels are caused to bend outside the load zone.
Another problem with Lau et al. concerns the location of the photo switches (1)-(4) (see FIG. 1 thereof) which communicate with the microprocessor (22) so that the microprocessor knows when to begin and when to end taking and recording loading and temperature readings for a specific panel traveling through the panel tester. Lau et al. disclose that a composite panel (10) moves in an xe2x80x9cSxe2x80x9d shaped path through the tester. The first deflector roll (14) is positioned midway between a first pair of spaced positioning rolls (13) each of which cooperates with its respective reaction roll (50) to clamp the panel (10) therebetween, all of which function to bend the panel in a first direction in the first curved portion of the xe2x80x9cSxe2x80x9d shaped path. The second deflection roll (16) is positioned substantially midway between a second pair of positioning rolls (13) each of which cooperates with its respective reaction roll (60) to clamp the panel (10) therebetween, all of which function to bend the panel in a second direction opposite to the first direction in the second curved portion of the xe2x80x9cSxe2x80x9d shaped path, i.e., in a reverse curvature to that forced by the first deflection roll (14). According to Lau et al., when the photo switches indicate that a panel is in the tester, readings from the load cells (18) and temperature sensor (24) are taken at predetermined intervals and the microprocessor uses these readings to calculate a strength and stiffness value for each panel tested. As shown and described in Lau et al., the photo switches are placed along the processing line with no particular regard as to how their placement may affect the calculated strength and stiffness values. In other words, what Lau et al. fail to recognize, and what has been determined according to the present invention, is that the location of the photo switches or sensors relative to the load zone of the xe2x80x9cSxe2x80x9d shaped path is important in terms of the overall calculated strength and stiffness value for each panel tested.
According to the present invention, it has been determined that in order to compute more accurate strength and stiffness values for the panels, each panel should be subjected to bending forces in the first and second curved portions of the xe2x80x9cSxe2x80x9d shaped path or load zone between the pairs of opposed positioning and reaction rolls adjacent to the respective deflector rolls. Any forces or adverse bending moments applied to the panels outside the load zone which causes the panels to bend in an undesirable manner, will result in less than accurate strength and stiffness values. Accordingly, since the panels should only be subjected to the appropriate bending forces within the load zone, and since the microprocessor calculates a strength and stiffness value for each panel traveling through the panel tester, it is desirable for the microprocessor to take and record the desired measurement readings only when each panel is in or substantially in the load zone of the xe2x80x9cSxe2x80x9d shaped path as defined between the pairs of opposed positioning and reaction rolls. Locating the photo switches as illustrated in Lau et al. results in the microprocessor taking and recording the load and temperature readings for the panels when the panels are not properly in the defined load zone of the xe2x80x9cSxe2x80x9d shaped path, thereby undesirably skewing the calculated strength and stiffness values for the panels.
Yet another problem with Lau et al. is that the panel tester and grader does not provide a mechanism to measure the thickness of each panel tested with a high degree of accuracy. As explained in Lau et al., a thickness value for the panels is needed in order to calculate the strength and stiffness values for the panels. In the preferred embodiment of Lau et al., a nominal thickness value for a set of panels (see, e.g., TABLES I and II therein and the description thereof) is simply inputted into the microprocessor, so that the appropriate calculations can be made. As noted, Lau et al. do teach that if a more accurate calculation of strength and stiffness is desired, a thickness sensor such as a laser sensor or an ultrasonic sensor may be used to measure the actual thickness instead of using the nominal thickness of each panel. Even so, what Lau et al. fail to recognize, and what has been determined according to the present invention, is that the thickness of each panel is a very significant parameter in determining the most precise measure of the strength and stiffness value for each tested panel. For example, a laser sensor will only measure the thickness of a panel at the specific location where the laser contacts the panel. As can be appreciated by those skilled in the art, panels of the type described herein can have varying thicknesses over the length and width of each panel. A single laser sensor cannot take into account the varying thicknesses throughout the panels. As a result, the averaged thickness measurement obtained by a laser sensor may not be a true representative measurement of the overall thickness of the particular panel. It is possible that multiple laser sensors could be used to improve the accuracy of the averaged thickness measurement for each panel, but multiple sensors would add undesirable cost and complexity to the overall panel tester, thereby resulting in a less than optimum machine. Likewise, an ultrasonic sensor will simply not provide accurate thickness measurements. As can be appreciated by those skilled in the art, panels of the type described herein have a tendency to vibrate as they are processed along the continuously operating panel tester and grader. Such vibrations in the panels will undoubtedly adversely affect the readings taken by an ultrasonic thickness tester. Thus, according to the present invention, it has been determined that in order to obtain a more accurate calculated strength and stiffness value for each panel, a new and improved thickness measuring device is required.
In sum, what is needed is a panel tester and grader that improves on the apparatus and method described in Lau et al., thereby providing a more accurate account of the strength and stiffness properties of each panel tested.
The present invention provides a panel tester and grader that accomplishes the features described herein as well as other features while at the same time alleviating the noted problems and other problems of the prior art. In one aspect, the present invention is an improvement over the apparatus and method of Lau et al. The noted advantages and other advantages of the present invention are realized in one aspect thereof in a panel tester and grader which provides a fully automatic structural-use panel performance test and grade system, and which also provides timely and tamper-free quality control testing. As such, the panel tester and grader system hereof provides reliable strength and stiffness testing and grading of product quality, heretofore unheralded in the prior art. The system in accordance with the present invention is particularly suited for continuous non-destructive in-line testing of wood panels. The system automatically applies a load to panels to be tested, preferably to deflect each panel a predetermined amount, reads and records the load required to deflect each panel, measures the thickness and temperature of each panel, all without operator involvement, and provides a printout test report which includes a strength and stiffness value for each tested panel. The system is extremely cost effective to the manufacturer as well as the ultimate user. Savings are realized, for example, in the ability to correct quality performance problems directly after they arise, thereby getting the most value as well as quality out of the processed panels. If the panel tester and grader of the present invention identifies poor quality panels, adjustments can be made to the upstream panel processing equipment so as to improve the quality of the finished panel products, thereby enabling the overall panel making process to operate in an efficient and economical manner which ultimately contributes to the overall realized profits.
In one aspect, the present invention prevents or substantially minimizes unwanted bending forces from being applied to the panels as the panels travel through the panel tester and grader. Like Lau et al., the present invention imposes a double reverse bend or xe2x80x9cSxe2x80x9d shaped configuration on the panels as they pass through the conveyor at line speed, and the loads and the amount of deflection required to form this xe2x80x9cSxe2x80x9d shaped configuration are used to determine the strength and stiffness values of the panels. Like Lau et al., the panel tester and grader according to the present invention allows panels to be graded so rejects can be identified and panels can be separated into grade groups representing different strength and stiffness ranges. Like Lau et al., the panels may be tested at one temperature, approaching the press temperature, and the stiffness and strength values are determined for the end products at another temperature. There are other similarities between the present invention and Lau et al. which can be observed from a comparison of one to the other. However, as will be further explained below, there are many differences between the present invention and Lau et al. such as, for example, the manner in which the positioning and reaction rolls are located in a predetermined position prior to sending the panels therebetween. One particular difference between the present invention and Lau et al. resides in the elimination of the first in-feed roll and the last out-feed roll and the problems attributable thereto, so as to provide more accurate strength and stiffness values for the tested panels. As a result, according to one embodiment of the present invention, panels are fed to a pair of opposed positioning and reaction rolls which represent the beginning of the first curve of the xe2x80x9cSxe2x80x9d shaped path or the beginning of the load zone without substantially subjecting the panels to a premature bending force which, if present, could undesirably affect the overall strength and stiffness value for each panel. Additionally, the present invention allows the panels to exit out of the panel tester and grader from between a pair of opposed positioning and reaction rolls which represent the end of the second curve of the xe2x80x9cSxe2x80x9d shaped path or the end of the load zone without substantially subjecting the panels to an extra, unnecessary bending force which, if present, could also undesirably affect the strength and stiffness value for each panel.
In another aspect of the present invention, sensors are strategically positioned along the processing line to prevent or to substantially minimize the taking and recording of unwanted load and temperature readings by the microprocessor. As noted, panels move in an xe2x80x9cSxe2x80x9d shaped path through the panel tester and grader. A first load roll is positioned generally midway between a first pair of spaced positioning rolls each of which cooperates with a respective reaction roll to clamp each panel therebetween, all of which function to bend each panel in a first direction in a first curved portion of the xe2x80x9cSxe2x80x9d shaped path or load zone. A second load roll is positioned generally midway between a second pair of spaced positioning rolls each of which cooperates with a respective reaction roll to clamp each panel therebetween, all of which function to bend each panel in a second direction opposite the first direction in a second curved portion of the xe2x80x9cSxe2x80x9d shaped path or load zone. The positioning and reaction rolls are advantageously located one above the other such that a vertical or substantially vertical plane extends through the centers of the respective opposing rolls. The planes extending through the centers of the rolls define nip areas between the opposing rolls and further define the beginning and ending boundaries of the curved portions of the xe2x80x9cSxe2x80x9d shaped path or load zone. A feature of the present invention involves the taking and recording of the load and temperature measurements of each panel when the panels are traveling within or substantially within the load zone. Thus, according to the present invention, it is desirable to properly position the necessary sensors as close as is practically possible to the planes extending through the opposed positioning and reaction rolls, thereby, in effect, being as close as possible to the boundaries of the curved portions.
In one embodiment, the positioning and reaction rolls are mounted for rotation about respective shafts. Each roll contains a circular groove which is preferably located midway between the ends of the roll, and which preferably has a depth which extends through the outer surface of the roll to the outer surface of the shaft. The positioning rolls are located relative to its opposing reaction roll such that the grooves of the positioning rolls align with the grooves of the respective reaction rolls, thereby providing a channel extending between the outermost vertical peripheries for each set of opposed rolls. A plurality of sensors, one for each channel, are positioned along the processing path traveled by the panels such that each sensor emits a signal which travels through its complementary channel. In this way, as a panel traveling through the processing line breaks the plane of the signal of any particular sensor, that sensor sends a signal to the microprocessor indicating that the sensor plane has been broken, whereby the microprocessor knows whether or not a panel is properly within or substantially properly within the load zone of the xe2x80x9cSxe2x80x9d shaped path. The sensors and microprocessor are programmed to cooperate together such that the microprocessor begins taking and recording load and temperature readings when certain sensor planes are broken thereby indicating that a panel is properly within the load zone, and stops taking and recording load and temperature readings when the other sensor planes are broken thereby indicating that a panel is not properly within the load zone. Since the planes extending through the opposing positioning and reaction rolls represent the beginning and ending points of the curved portions of the xe2x80x9cSxe2x80x9d shaped path or load zone, and since the sensors pass sensing signals or mediums through the grooves or channels extending between the respective opposing rolls near the outer diameter of the shafts of the rolls, the load and temperature readings are only taken and recorded while the panels are substantially within the load zone. This is an improvement over the Lau et al. reference because, unlike Lau et al., the panel tester and grader according to the present invention includes sensors which are strategically placed along the panel processing line with respect to the load zone so as to communicate with a microprocessor in such a manner that prevents or substantially minimizes the taking of undesirable load and temperature readings. Contrary to the present invention, the photo switches of Lau et al. are located in areas far removed from the intended load zone of the xe2x80x9cSxe2x80x9d shaped path.
In a further aspect of the present invention, a panel thickness measuring device is positioned relative to the framework of the panel tester and grader in order to provide a more accurate measurement of the thickness of each panel as the panels are fed through the machine. As mentioned, the thickness of each panel contributes to the final calculated strength and stiffness value for each panel. Thus, the more accurate the thickness measurement is for each panel, the more accurate the strength and stiffness values will be for each panel. The positioning and reaction rolls are supported by suitable framework which may be moved in a vertical direction through connection with electro-mechanical actuators. The electro-mechanical actuators move the appropriate portions of the framework, thereby moving the rolls, into a preset position depending on the general thickness of the panels to be tested. Preferably, the gap between each pair of cooperating rolls will be slightly smaller than the thickness of the panels to be tested. The electro-mechanical actuators are preferably provided with spring mounts so that when a panel passes between the cooperating rolls, the actuators can absorb the difference in the thickness of each panel while maintaining the rolls in contact with the adjacent face of the panel being tested. Thus, at least portions of the framework has, in effect, a limited range of motion during operation which enables panels having varying thicknesses to pass through the machine so that the panels are not damaged when passing between the opposed sets of rolls. According to the present invention, a thickness measuring device such as a LVDT is coupled to the framework so as to be able to measure the distance between the cooperating sections of the framework as this distance varies, according to the thickness of each panel traveling through the machine. A thickness measuring device according to the prevent invention will pick up most, if not all, of the variations or aberrations of thickness in each panel to be tested. In this way, the microprocessor is able to calculate a more accurate averaged thickness measurement for each panel which will result in a more accurate overall strength and stiffness value.
In yet another aspect, the present invention provides a method of testing the strength and stiffness characteristics of panel-like materials comprising the steps of feeding each panel in an xe2x80x9cSxe2x80x9d shaped load zone between a plurality of pairs of rolls, deflecting each panel from both sides and measuring the deflection load for a deflection amount when the panel is substantially within the load zone, measuring the temperature of the panel being tested when each panel is substantially within the load zone, measuring the actual thickness of each panel when each panel is substantially within the load zone, providing a plurality of sensors which are strategically placed along the processing line which determine when each panel is substantially within the load zone and, preferably, calculating an end-use strength and stiffness value for each panel tested based on the load, deflection, temperature and thickness readings for each panel.
Other features and advantages of the invention will become apparent to those skilled in the art upon review of the following detailed description, claims and drawings in which like numerals are used to designate like features.